This invention relates to the production of insulated metallic strip and, more particularly, to the production of insulated metallic strip suitable for use in the coils of power transformers. Strip is typically, any material whose width is at least 30 times greater than the thickness.
Power transformers, such as overhead distribution transformers and padmounted distribution transformers, generally include coils which are wound from relatively wide flat metallic strips of conductive material such as aluminum. Heretofore, the aluminum strips have been produced by first casting aluminum into ingots and then cold rolling and hot rolling the ingots to form sheets which are then slit to form the strips. In addition, the strips have been subjected to secondary metal treating processes to contour the edges thereof. Contoured edges enable the strips to be insulated with a dielectric in an optimal manner. More recently, there has been developed an improved process for the production of continuous metal strip employing continuous extrusion techniques to continuously form flat metal strip suitable for producing coils for power transformers. Such improved method known as the "Conform" process and the apparatus for practicing the Conform process is disclosed in U.S. Pat. Nos. 5,359,874 and 5,406,818. Briefly, in that process, twin metal billets are fed to dual circumferential grooves formed in a rotating wheel. The billets are advanced first to a wedge-shaped gap which deforms the billets and they are then advanced to a die. The die has a die opening with a circumferentially discontinuous, annular cross-section. The metal from each billet merges in the die opening and exits therefrom in the form of a slit tube. The tube is then opened and flattened to form a flat strip by advancing the slit tube over a forming member having a progressively increasing width.
In manufacturing transformer coils, the coils are wound from continuous metal strip with interleaved layers of insulation material. In conventional transformers the insulation material are layers of insulating paper. The paper provides electrical insulation between the layers or turns of the wound conductor material. An adhesive is applied to the paper before it is wound into the transformer and the wound transformer is thereafter heated and the adhesive on the paper binds the metallic conductor turns and the paper turns together into a solid unit. The paper is typically five mils thick and thus adds considerably to the size and bulk of a transformer having many turns. This increase in size requires a larger housing for the transformer and also adds to the cost of manufacturing, shipping and installing the transformers.
It would be desirable to have strip conductors for winding into the coils of the transformer where the strip conductors are provided with an insulating coating so as to eliminate the need for the paper layers between the metallic layers of the coil. Such an arrangement has been disclosed in U.S. Pat. No. 5,528,820. In that patent there is disclosed a stock material for winding into a magnet coil which includes a running length of conductive aluminum metal having a cross section that has first and second long sides and two short sides. The long sides and the short sides meet at corners which are substantially free of jagged edges and sharp corners. An insulating epoxy polymer coating about 0.001" thick uniformly covers the first long side and the short sides, with the second long side being substantially free of the insulating coating. A heat-activatable adhesive is arranged non-continuously on one of the long sides. While this type of strip conductor for transformers has eliminated the need for the intervening layer of paper strip material, it has left something to be desired.
It would be desirable to provide a flat metallic strip of conductive material with rounded edges for winding into a magnet coil having an insulating coating on one side and on the pair of edges of the strip and extending onto the second side of the strip from each of the edges a distance at least as great as to prevent turn to turn discharge. This distance is critical and provides increased turn to turn "creep" distance where creep is defined as the discharge of electricity across a solid surface. It would also be desirable to provide a plurality of spaced areas of the insulating coating on the second side between the insulating coating extending around the edges to allow the strip to wind into coils without "dishing", sometimes referred to as "canoeing", and the bare stripes between the insulation coating allow the strip to be cold pressure welded to leads and taps without the need to remove the coating. The term "dishing" is defined as the occurrence of a non-flat surface across the outside cylindrical surface of the wound coil due to non-uniform thickness of the individual strip material windings. "Dishing" occurs when the edges of the strip are thicker than the middle and when the strip is wound into a tight coil, the coil has a larger diameter at its sides than it does in the middle. The edges of the strip will be stretched and when unwound the strip will have wavy edges.
It would also be desirable to produce the flat insulated strip in a continuous process where the insulating coating is applied immediately following the production of the flat metallic strip and there is little chance for contamination of the strip which is critical for the application of thin coatings.